In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) is heretofore used. Specifically, the PS plate is exposed through a mask, for example, a lith film, and then subjected to development processing, for example, with an alkaline developer to remove the unnecessary image-recording layer corresponding to the non-image area by dissolving while leaving the image-recording layer corresponding to the image area, thereby obtaining the lithographic printing plate.
Due to the recent progress in the technical field, nowadays the lithographic printing plate can be obtained by a CTP (computer-to-plate) technology. Specifically, a lithographic printing plate precursor is directly subjected to scanning exposure using laser or laser diode without using a lith film and developed to obtain a lithographic printing plate.
With the progress described above, the issue on the lithographic printing plate precursor has transferred to improvements, for example, in image-forming property corresponding to the CTP technology, printing property or physical property. Also, with the increasing concern about global environment, as another issue on the lithographic printing plate precursor, an environmental problem on waste liquid discharged accompanying the wet treatment, for example, development processing comes to the front.
In response to the environmental problem, simplification of development or plate making or non-processing has been pursued. As one method of simple plate making, a method referred to as an “on-press development” is practiced. Specifically, according to the method after exposure of a lithographic printing plate precursor, the lithographic printing plate precursor is mounted as it is on a printing machine without conducting conventional development and removal of the unnecessary area of image-recording layer is performed at an early stage of printing process.
Also, as a method of simple development, a method referred to as a “gum development” is practiced in which removal of an overcoat layer and the unnecessary area of image-recording layer and protection of a hydrophilic surface revealed by the removal are performed at the same time and in which the removal of an overcoat layer and the unnecessary area of image-recording layer is performed in the same bath using not a conventional highly alkaline developer but a finisher or gum solution of near-neutral pH.
In the simplification of plate making operation as described above, a system using a lithographic printing plate precursor capable of being handled in a bright room or under a yellow lamp and a light source is preferred from the standpoint of workability. Thus, as the light source, a semiconductor laser emitting an infrared ray having a wavelength of 760 to 1,200 or a solid laser, for example, YAG laser, is used. An UV laser is also used.
As the lithographic printing plate precursor capable of undergoing on-press development, for instance, a lithographic printing plate precursor having provided on a hydrophilic support, an image-recording layer (heat-sensitive layer) containing microcapsules having a polymerizable compound encapsulated therein is described in Patent Documents 1 and 2. A lithographic printing plate precursor having provided on a support, an image-recording layer (photosensitive layer) containing an infrared absorbing agent, a radical polymerization initiator and a polymerizable compound is described in Patent Document 3. A lithographic printing plate precursor capable of undergoing on-press development having provided on a support, an image-recording layer containing a polymerizable compound and a graft polymer having a polyethylene oxide chain in its side chain or a block polymer having a polyethylene oxide block is described in Patent Document 4.
In case of using the polymerization reaction, it is known as a usual practice to provide an overcoat layer (also referred to as a protective layer) having an oxygen blocking property on the image-recording layer in order to prevent polymerization inhibition due to oxygen in the air and to increase sensitivity and printing durability. It is conventionally known that a water-soluble resin, for example, polyvinyl alcohol is used in the overcoat layer for the purpose described above. A lithographic printing plate precursor of on-press development type provided with an overcoat layer using a modified polyvinyl alcohol is described in Patent Document 5. Also, a lithographic printing plate precursor provided with an overcoat layer using an inorganic stratiform compound having an oxygen blocking property, for example, mica and a water-soluble resin is described in Patent Document 6.
However, in the on-press development system, when the overcoat layer mainly composed of polyvinyl alcohol as described in Patent Document 5 is used, there is a problem in that interfacial mixing partially arises at the coating and drying of the overcoat layer and a water-soluble resin is mixed into the image-recording layer. Due to the mixing of a water-soluble resin into the image-recording layer, the water-soluble resin remains on the surface of image-recording layer after the on-press development to cause a drawback of inferior ink receptivity at the printing after the on-press development.
Moreover, when the water-soluble resin even reaches an undercoat layer, polymerization arising from photopolymerization becomes insufficient to cause a drawback of insufficient printing durability.
In general, in the gum development in which an overcoat layer and the unexposed area of image-recording layer are removed at the same time with a gum solution of low pH, it takes a time for removing the overcoat layer to cause problems in that development removability of the unexposed area of image-recording layer is poor and in that since the component of overcoat layer once removed is difficult to be stably dispersed in the developer, the component of overcoat layer precipitates in the developing tank and in case of running processing (continuous development by an automatic development machine), the precipitate adheres on the lithographic printing plate precursor during processing as development scum to be liable to cause image defect.